1. Field of the Invention
The present invention relates to a backlight module. More particularly, the present invention relates to a light guide plate (LGP) applied to a backlight module.
2. Description of Related Art
FIG. 1 is a schematic cross-sectional view of a conventional backlight module, FIG. 2 is a schematic view of transmission paths of light in an LGP of FIG. 1, and FIG. 3 is a diagram showing the relationship between a light emitting angle of a light emitting surface of the LGP of FIG. 1 and the luminance thereof. Referring to FIG. 1, the backlight module 100 includes a light source 110, an LGP 120, and a reflective sheet 130. The light source 110 is disposed adjacent to a light incident surface 122 of the LGP 120, and the reflective sheet 130 is disposed on a bottom surface 124 of the LGP 120. The light provided by the light source 110 travels into the LGP 120 through the light incident surface 122, and travels out of the LGP 120 through a light emitting surface 126 of the LGP 120, so as to form a plane light source, and then travels to a liquid crystal display panel (LCD panel) (not shown) on the light emitting surface 126. The reflective sheet 130 may reflect the light to the light emitting surface 126. A plurality of grooves 128 on the bottom surface 124 of the LGP 120 may change the transmission paths of the light, so that the light travels to the light emitting surface 126 at a smaller incident angle. Therefore, the luminance efficiency of the LGP 120 is improved.
In detail, referring to FIG. 2, when a material of the LGP 120 is acrylic, a critical angle at which the light is totally reflected from the interface between the LGP and the air as the light is transmitted from the LGP to the air is about 42 degrees. For example, the section of each groove 128 is in V-shaped with two opposite sides equal in length and has a vertex angle α1 of 90 degrees. When the light L1 travels to a slanted surface 128a of one of the grooves 128 at an incident angle θ1 of 45 degrees, the light L1 is totally reflected at the slanted surface 128a of the groove 128. After that, the light L1 travels to the light emitting surface 126 at an incident angle of 0 degree, and travels to the LCD panel through the light emitting surface 126.
Moreover, when the light L2 travels to the slanted surface 128a of the groove 128 at an incident angle θ2 of 20 degrees, the light L2 will be transmitted to the groove 128 through the slanted surface 128a of the groove 128. At this time, a refraction angle θ2a of the light L2 is about 30.7 degrees. Then, the light L2 travels to another slanted surface 128b of the groove 128 at an incident angle θ2b of 59.3 degrees, and passes through the slanted surface 128b of the groove 128 to be transmitted into the LGP 120. At this time, a refraction angle θ2c of the light L2 is about 35.1 degrees. After that, the light L2 travels to the light emitting surface 126 at an incident angle θ2d of 80.1 degrees, and is totally reflected at the light emitting surface 126.
In addition, when the light L3 travels to the bottom surface 124 at an incident angle θ3 of 65 degrees, the light L3 will be totally reflected at the bottom surface 124. Then, the light L3 travels to the slanted surface 128a of the groove 128 at an incident angle θ3a of 70 degrees, and is totally reflected at the slanted surface 128a of the groove 128. After that, the light L3 travels to the light emitting surface 126 at an incident angle θ3b of 25 degrees, and travels to the LCD panel through the light emitting surface 126. At this time, a refraction angle θ3c of the light L3 is about 39.2 degrees.
It should be noted that since the light L3 passing through the light emitting surface 126 has a greater refraction angle θ3c, after the light provided by the light source 110 passes through the LGP 120 to form the plane light source, the concentration of the light is not satisfactory. Referring to FIG. 3, when the light provided by the light source 110 passes through the LGP 120 to form the plane light source, the half visual angles are within the range between −25.792 and 17.134 degrees. Here, each half visual angle is a light emitting angle corresponding to the luminance which is a half of the maximum luminance.